We're evaluating the BCM4334x platform for use in a real time streaming application over 802.11n. We're noticing a 70-millisecond pause in data traffic every 120 seconds. The pause always follows a Clear to Send signal.

What causes this pause every 120 seconds? How can we configure the interval or disable the behavior?

1. Configure the device as an access point. Our host environment is Linux, so we can run an access point using hostapd. We see the behavior when running 802.11n at 2.4 or 5 Ghz, for example.

2. I next connect to the device's WiFi network from my Mac. I start a capture in Wireshark with the interface set to capture in monitor mode (Capture->Options, double-click the WiFi interface, then check "capture packets in monitor mode").

3. Start a capture. After 5 minutes, look for Clear-to-send frames (perhaps after filtering on the mac address of the device to make life easier). We see Clear-to-send frames every 120 seconds. They have a duration of 10000 microseconds and are addressed to the device. They do not follow a Request-to-send (perhaps they are CTS-to-self?).

5. Device1 will send data to Device2. Start an iPerf server on Device2 with

> iperf -s -u -i 1

6. On Device1, start sending 300 seconds of UDP traffic with

> iperf -c 192.168.42.31 -u -b 250k -l 250 -t 300

7. After 5 minutes, stop the test. Find CTS frames that specify duration=10000 microseconds using a Wireshark filter like: "(wlan.addr == <mac address Device1> or wlan.addr == <mac address Device2>) and wlan.duration == 10000". When I do this, I see CTS frames from each device, where the frames at each device are separated by 120 seconds.

8. Now, select one of the CTS frames in Wireshark and then clear the filter. This brings back the surrounding data frames. We notice that data packets are usually separated by about 8 ms; after one of these CTS frames, however, there's a 60 to 80 ms delay before the next data frame.

Could these delays be due to power save, or some sort of periodic off-channel scan?